Light emitting device

ABSTRACT

A light emitting device is provided. The light emitting device includes a light emitting element, which emits blue light, and a light transmissive member having a first principal face bonded to the light emitting element and a second principal face opposite the first principal face. The light transmissive member has a light transmissive base material and wavelength conversion substances, which are contained in the base material and which absorb the light from the light emitting element and emit light. The wavelength conversion substances are localized in the base material towards the first principal face, and include a first phosphor which emits green to yellow light and a second phosphor which emits red light. The first phosphor is more localized towards the first principal face than the second phosphor. The second phosphor is a manganese-activated fluoride phosphor.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application of U.S. patent application Ser. No.15/479,854, filed Apr. 5, 2017, which claims the benefit of JapanesePatent Application No. 2016-076811, filed on Apr. 6, 2016, thedisclosure of both which are hereby incorporated by reference in theirentirety.

BACKGROUND

The present disclosure relates to a light emitting device.

Japanese Unexamined Patent Application Publication No. 2010-251621, forexample, discloses a semiconductor light emitting device comprising afirst light emitting composition made by sealing a first phosphor, whichis a low refractive index magnesium-activated fluoro complex phosphor,with a first transparent sealing material; a second light emittingcomposition made by sealing a second phosphor having a higher refractiveindex than that of the first transparent sealing material with a secondtransparent sealing material, which has a refractive index higher thanthat of the first sealing member, but equal to or lower than that of thesecond phosphor; and a semiconductor light emitting element, which isthe light source for exciting the first phosphor and the secondphosphor. The patent application publication also discloses that thefirst light emitting composition and the second light emittingcomposition are arranged so that the light released from the secondphosphor is extracted from the light emitting device via the first lightemitting composition.

SUMMARY

However, when the first light emitting composition is provided as theoutermost layer as disclosed in the aforementioned patent applicationpublication, the fluoro complex phosphor, which has relatively poorresistance to environmental factors, is likely to easily degrade,thereby reducing the reliability of the light emitting device.

Accordingly, an object of an embodiment of the present disclosure is toprovide a highly reliable light emitting device with reduced degradationof a manganese-activated fluoride phosphor.

A light emitting device according to an embodiment of the presentdisclosure is characterized by comprising a light emitting element,which emits blue light, and a light transmissive member having a firstprincipal face bonded to the light emitting element and a secondprincipal face opposite the first principal face. The light transmissivemember has a light transmissive base material and wavelength conversionsubstances contained in the base material to absorb the light from thelight emitting element and emit light. The wavelength conversionsubstances are localized in the base material towards the firstprincipal face and include a first phosphor, which emits green to yellowlight, and a second phosphor, which emits red light. The first phosphoris more localized towards the first principal face than the secondphosphor. The second phosphor is a manganese-activated fluoridephosphor.

The light emitting device according to an embodiment of the presentdisclosure can be a highly reliable light emitting device with reduceddegradation of a manganese-activated fluoride phosphor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic perspective view of a light emitting deviceaccording to an embodiment of the present disclosure.

FIG. 1B is a schematic sectional view of the light emitting device inFIG. 1A.

FIG. 2A is a schematic sectional view of a wavelength conversion sheetaccording to the embodiment in FIGS. 1A and 1B.

FIG. 2B is a schematic sectional view of another wavelength conversionsheet according to the embodiment in FIGS. 1A and 1B.

FIG. 3 is a graph showing the changes in emission chromaticity in agingtests conducted on light emitting devices according to an example of thepresent disclosure and a comparative example.

FIG. 4A is a graph showing changes in the chromaticity value x in reflowtests conducted on light emitting devices according to an example of thepresent disclosure and a comparative example.

FIG. 4B is a graph showing changes in the chromaticity value y in reflowtests conducted on light emitting devices according to an example of thepresent disclosure and a comparative example.

DETAILED DESCRIPTION

Certain embodiments of the present disclosure will be explained belowwith reference to the accompanying drawings. The light emitting devicesexplained below are intended to embody the technical ideas of thepresent disclosure, and unless otherwise specifically noted, the presentdisclosure is not limited to those described below. Moreover, the sizesand relative positions of the members shown in the drawings might beexaggerated for clarity of explanation.

It is assumed that the visible spectrum has the wavelengths rangingbetween 380 nm and 780 nm, the blue region has a wavelength rangingbetween 420 nm and 480 nm, the green region has a wavelength rangingbetween 500 nm and 570 nm, the yellow region has a wavelength range thatis above 570 nm but 590 nm at most, and the red region has a wavelengthranging between 610 nm and 750 nm.

An Embodiment of Present Disclosure

FIG. 1A is a schematic perspective view of a light emitting device 100according to an embodiment of the present disclosure. FIG. 1B is aschematic sectional view of the light emitting device 100.

In FIGS. 1A and 1B, the horizontal direction, the vertical direction,and the front to back (depth) direction are shown as the X direction,the Y direction, and the Z direction, respectively. Each of the X, Y,and Z directions (axes) is perpendicular to the other two directions(axes). More specifically, the direction to the right is the X₊direction, to the left is the X⁻ direction, the upward direction is theY₊ direction, the downward direction is the Y⁻ direction, the directiontowards the front is the Z₊ direction, and towards the back is the Z−direction. The Y⁻ direction is the mounting direction for the lightemitting device 100. The Z₊ direction is the principal emissiondirection of the light emitting device 100. The X, Y, and Z directions(axes) in FIGS. 2A and 2B, which are described later, correspond to theX, Y, and Z directions (axes) in FIGS. 1A and 1B. Hereinafter, the faceof each constituent element of the light emitting device 100perpendicular to the Z₊ direction is referred to as the “front face,”and that is perpendicular to the Z⁻ direction is referred to as the“rear face.”

As shown in FIGS. 1A and 1B, the light emitting device 100 includes alight emitting element 10 and a light transmissive member 20. The lightemitting device 100 further includes a light guide member 60, conductivebonding members 70, and a wiring board 80. The light emitting element 10emits blue light. The light emitting element 10 is bonded to the wiringboard 80 via the conductive bonding members 70. The light transmissivemember 20 has a first principal face 20 a bonded to the light emittingelement 10 and a second principal face 20 b opposite the first principalface 20 a. The bonding between the light emitting element 10 and thelight transmissive member 20 is achieved via the light guide member 60.The light transmissive member 20 has a light transmissive base material30 and wavelength conversion substances 40 contained in the basematerial 30. The wavelength conversion substances 40 absorb the lightfrom the light emitting element 10 and emit light. The wavelengthconversion substances 40 are localized in the base material 30 towardsthe first principal face 20 a. The wavelength conversion substances 40include a first phosphor 41, which emits green to yellow light, and asecond phosphor 42, which emits red light. The first phosphor 41 islocalized closer to the first principal face 20 a than the secondphosphor 42. The second phosphor 42 is a manganese-activated fluoridephosphor.

In the light emitting device 100 constructed as above, the ratio of thevolume occupied by the wavelength conversion substances 40 to the basematerial 30 of the light transmissive member 20 is higher on the firstprincipal face 20 a side, which is located on the inner side of thedevice than on the second principal face 20 b side, which is closer tothe outside. Among the wavelength conversion substances 40, the ratio ofthe volume occupied by the first phosphor 41 is higher than that of thesecond phosphor 42 on the first principal face 20 a side, while theratio of the volume occupied by the second phosphor 42 is higher thanthat of the first phosphor 41 on the second principal face 20 b side. Inthis manner, the second phosphor 42, which is a manganese-activatedfluoride phosphor, is distributed in the middle portion of the lighttransmissive member 20 in the thickness direction, i.e., in the zdirection. Accordingly, the second phosphor 42 can be readily protectedagainst the external environment by the base material 30 located closerto the second principal face 20 b than the second phosphor 42. Moreover,the first phosphor 41 located closer to the first principal face 20 athan the second phosphor 42 can moderate the irradiation of the lightemitting element 10 against the second phosphor 42, which is amanganese-activated fluoride phosphor that can easily reach saturationin terms of emission efficiency, so as not to be excessive to therebyreduce degradation of the second phosphor 42. In this manner, a highlyreliable light emitting device that can reduce degradation ofmanganese-activated fluoride phosphor can be achieved.

A preferred embodiment of the light emitting device 100 will beexplained in detail below.

As shown in FIGS. 1A and 1B, the light emitting device 100 furtherincludes a cover member 50 having optical reflectance. The cover member50 preferably covers the lateral faces 20 c located between the firstprincipal face 20 a and the second principal face 20 b. This can preventthe light transmissive member 20 from being exposed to the externalenvironment, thereby reducing the degradation particularly of the secondphosphor 42. Moreover, since the reduced lateral emission from the lighttransmissive member 20 promotes the emission and the heat generation ofthe wavelength conversion substances 40 and can promote the degradationof the wavelength conversion substances 40, particularly the secondphosphor 42, the construction of this embodiment is technically highlysignificant. From the same perspective, the cover member 50 preferablyalso covers the lateral faces of the light emitting element 10. Thecover member 50 is constructed with a base material 51 and a whitepigment 55 contained therein.

As shown in FIG. 1B, the light emitting element 10 includes asemiconductor stack 13 and an electrically insulating light transmittivesubstrate 15. The substrate 15 has a third principal face 15 a on whichthe semiconductor stack 13 is disposed and a fourth principal face 15 bopposite the third principal face 15 a. In such a case, the firstprincipal face 20 a of the light transmissive member is preferablybonded to the fourth principal face 15 b of the substrate. The lightemitting element 10 having a substrate 15 can achieve a relatively highproduction efficiency, as well as easily constructing a flip-chip typeemission structure without requiring wire bonding, thereby increasingthe optical coupling efficiency into the light transmissive member 20,and light extraction efficiency.

As shown in FIG. 1B, the light transmissive member 20 preferablyincludes a first layer 201 composed of the base material 30 and thefirst phosphor 41, a second layer 202 composed of the base material 30and the second phosphor 42, and a third layer composed of the basematerial 30 in that order from the first principal face 20 a to thesecond principal face 20 b. This clearly segments the light transmissivemember 20 into layers in which the first phosphor 41 and the secondphosphor 42 are respectively distributed in the base material 30, whichcan clearly achieve the functionality and the effect of how the basematerial 30, the first phosphor 41, and the second phosphor 42 arearranged relative to one another as described above. The first layer201, the second layer 202, and the third layer 203 are preferablydirectly in contact with one another, but a base material 30 may beinterposed between the layers.

As shown in FIG. 1B, the base material 30 may include a first basematerial 31, a second base material 32, and a third base material 33,where the first layer 201 is composed of the first base material 31 andthe first phosphor 41, the second layer 202 is composed of the secondbase material 32 and the second phosphor 42, and the third layer 203 iscomposed of the third base material 33. In such a case, it is preferableto construct the second base material 32 with a material having arefractive index, which is the same as, or a lower than, the refractiveindex of the first base material 31, while constructing the third basematerial 33 with a material having a refractive index, which is the sameas that of the second base material 32 if it is lower than the firstbase material 31, or a material having a lower refractive index thanthat of the second base material 32. This can increase the lightextraction efficiency of the light emitting device 100.

From the perspective of affinity between the layers, moreover, it ispreferable to construct the entire base material 30 with the samematerial.

FIG. 2A is a schematic sectional view of a wavelength conversion sheet200 according to the embodiment in FIGS. 1A and 1B. FIG. 2B is aschematic sectional view of another wavelength conversion sheet 220according to the embodiment in FIGS. 1A and 1B. The light transmissivemember 20 used in the light emitting device 100 is obtained, forexample, by cutting the wavelength conversion sheet 200 or 220 intosmall pieces.

As shown in FIGS. 2A and 2B, the wavelength conversion sheet 200 has afirst principal face 20 a and a second principal face 20 b opposite thefirst principal face 20 a. The wavelength conversion sheet 200 has alight transmissive base material 30, and wavelength conversionsubstances 40 contained in the base material 30. The wavelengthconversion substances 40 absorb blue light, and emit light. Thewavelength conversion substances 40 are localized in the base material30 toward the first principal face 20 a. The wavelength conversionsubstances 40 include a first phosphor 41 which emits green to yellowlight, and a second phosphor 42 which emits red light. The firstphosphor 41 is localized closer to the first principal face 20 a thanthe second phosphor 42. The second phosphor 42 is a manganese-activatedfluoride phosphor.

The wavelength conversion sheet 200, in particular, can be produced, forexample, by pasting together a first sheet 201 composed of the firstbase material 31 and the first phosphor 41, a second sheet 202 composedof the second base material 32 and the second phosphor 42, and a thirdsheet 203 composed of the third base material 33 in that order. Thelight transmissive member 20 prepared by cutting such a wavelengthconversion sheet 200 into a small piece is clearly segmented into layersin which the first phosphor 41 and the second phosphor 42 arerespectively distributed in the base material 30, clearly achieving thefunctionality and the effect of how the base material 30, the firstphosphor 41, and the second phosphor 42 are arranged relative to oneanother. For such a wavelength conversion sheet 200, the concentrationand the distribution of the wavelength conversion substances 40, and thelayer thickness can be easily controlled. Thus, a homogeneous lighttransmissive member 20 can be easily produced.

The wavelength conversion sheet 220 shown in FIG. 2B can be produced,for example, by allowing the first phosphor 41 to settle first towardsthe first principal face 20 a, followed by allowing the second phosphor42 to settle above the first phosphor 41 during the steps of mixing thewavelength conversion substances 40 into the base material 30 in aliquid form, and curing the base material 30. At this time, in order toforcibly settle the first phosphor 41 and/or the second phosphor 42, acentrifugation method may be employed. The wavelength conversion sheet220 and the light transmissive members prepared by cutting the sheetinto pieces have the first phosphor 41 and the second phosphor 42 whichtend to be localized in the base material 30, while also having aportion where both phosphors are mixed together. This can create aconstruction that achieves the functionality and the effect of how thebase material 30, the first phosphor 41, and the second phosphor 42 arearranged relative to one another described above, while facilitating themixing of the colors of the light emitted by the first phosphor 41 andthe second phosphor 42.

Each constituent element of the light emitting device 100 will beexplained below.

Light Emitting Device 100

The light emitting device 100, for example, is a light emitting diode(LED). The light emitting device 100 described above is of a sideemission type, also referred to as “side view type,” but it can beadapted to be of a top emission type, also referred to as “top viewtype.” In a side emission type light emitting device, the mountingdirection is perpendicular to the principal emission direction. In a topemission type light emitting device, the mounting direction is parallelto the principal emission direction. The front view shape, i.e., theshape when viewed from the principal emission direction, of the lightemitting device can be suitably selected, but a rectangular shape ispreferable in terms of mass production efficiency. The front view shapein the case of a side emission type light emitting device, inparticular, is preferably a rectangle having longitudinal and traversedirections. In the case of a top emission type light emitting device, onthe other hand, a square shape is preferable. Moreover, it is good forboth the light emitting element and the light transmissive member tohave a similar front view shape to that of the light emitting device.Alternatively, the light emitting device may be of a chip size package(CSP) type, which does not include a wiring substrate, but instead haspositive and negative electrodes for the light emitting element, orprojected electrodes joined with the positive and negative electrodes,as the terminals for external connection. Examples of the projectedelectrodes include bumps and pillars.

Light Emitting Element 10

The light emitting element 10 at least includes a semiconductor stack,which configures a light emitting element structure, and in many casesfurther includes a substrate. Examples of the light emitting element 10include an LED chip. The front view shape of the light emitting element10 is preferably rectangular in shape, particularly a square or arectangle elongated in one direction (in X direction in FIG. 1A). Thelateral faces of the light emitting element 10 or the substrate thereofmay be perpendicular to the front face, or be inwardly or outwardlyoblique. The light emitting element 10 preferably has positive andnegative (p, n) electrodes on the same face. In the case where the lightemitting element 10 is of a flip-chip (face down) mounting type, theemission face, i.e., the front face, is one opposite the face on whichthe electrodes are formed. The light emitting element 10 may includepositive and negative electrodes and/or an insulation film. The positiveand negative electrodes can be constructed with gold, silver, tin,platinum, rhodium, titanium, aluminum, tungsten, palladium, nickel, orany of their alloys. The insulation film can be constructed with anoxide or a nitride of at least one element selected from the groupconsisting of silicon, titanium, zirconium, niobium, tantalum, andaluminum. The number of light emitting elements installed in a lightemitting device may be one or plural. A plurality of light emittingelements can be connected in series or parallel.

Semiconductor Stack 13

The stack 13 of semiconductor layers at least includes an n-typesemiconductor layer and a p-type semiconductor layer, and preferably anactive layer interposed therebetween. The peak emission wavelength ofthe light emitting element 10 can be selected from the ultravioletregion to the infrared region by varying the semiconductor materialand/or the mixed crystal ratio. For the semiconductor material, it ispreferable to use a nitride semiconductor capable of emitting shortwavelength light that can efficiently excite the wavelength conversionsubstances. A nitride semiconductor is expressed by the generalexpression, In_(x)Al_(y)Ga_(1-x-y)N (0≤x, 0≤y, x+y≤1). The peak emissionwavelength of the light emitting element 10 is preferably in the bluerange from the perspective of emission efficiency, excitation ofwavelength conversion substances, and color mixing, more preferably in arange between 450 nm and 475 nm. InAlGaAs-based semiconductors,InAlGaP-based semiconductors, zinc sulfide, zinc selenide, siliconcarbide or the like can also be used.

Substrate 15

A substrate for crystal growth capable of growing a semiconductorcrystal comprising a light emitting element 10 is primarily used for thelight emitting element substrate 15, but a substrate for bondingpurposes to be bonded to the semiconductor stack 13 separated from asubstrate for crystal growth may alternatively be used. A lighttransmissive substrate easily allows for the employment of flip-chipmounting as well as easily increasing the light extraction efficiency.Examples of the substrate include sapphire, spinel, gallium nitride,aluminum nitride, silicon, silicon carbide, gallium arsenide, indiumphosphide, zinc sulfide, zinc oxide, zinc selenide, diamond, and thelike. Among all, sapphire is preferable. The thickness of the substratecan be suitably selected, but from the perspective of the strength ofthe substrate and/or the thickness of the light emitting device 100, thethickness is preferably in a range between 0.02 mm and 1 mm, morepreferably in a range between 0.05 mm and 0.3 mm. It is fine to notprovide a light emitting element substrate.

Light Transmissive Member 20, Wavelength Conversion Sheet 200, 220

The light transmissive member 20 is a member disposed on the lightemitting element 10 to allow the light emitted from the light emittingelement 10 to transmit therethrough to the outside of the light emittingdevice 100. The light transmissive member 20 is constructed at leastwith a base material 30 and wavelength conversion substances 40contained in the base material 30, and can function as a wavelengthconversion member. For the light transmissive member 20, a sintered bodycomprising wavelength conversion substances 40 and an inorganicmaterial, for example, alumina, a plate-like crystal of a wavelengthconversion substance, or the like can alternatively be used.

Base Material 30 for the Light Transmissive Member 20

The base material 30 for the light transmissive member 20 can be anythat is light transmissive for the light emitted by the light emittingelement 10. The term “light transmissive” refers to the opticaltransmittance of the base material being preferably at least 60% at thepeak emission wavelengths of the light emitting element 10, morepreferably at least 70%, even more preferably at least 80%. The basematerial 30 can be constructed by using a silicone resin, epoxy resin,phenol resin, polycarbonate resin, acrylic resin, or any of theirmodified resins. Glass may alternatively be used. Among all,silicone-based resins, i.e., silicone or modified silicone resins, arepreferable because they are highly heat and light resistant. Specificexamples of silicone resins include dimethyl silicone resin,phenyl-methyl silicone resin, and diphenyl silicone resin. The basematerial 30 can be constructed with a single layer of one of thesematerials, or multiple layers of two or more of these materials. The“modified resins” herein include hybrid resins.

The resin or glass used for the base material 30 described above maycontain various fillers. Examples of fillers include silicon oxide,aluminum oxide, zirconium oxide, zinc oxide, and the like. One of thesefillers can be used singly, or two or more can be used in combination.Silicon oxide, which has a low thermal expansion coefficient, isparticularly preferable. Moreover, the use of nano particles as fillerscan increase scattering, including Rayleigh scattering, of the bluelight from the light emitting element, thereby reducing the amount ofthe wavelength conversion substances used. Nano particles are particleshaving particle sizes in a range between 1 nm and 100 nm. The “particlesize” herein is as defined by, for example, the D₅₀.

Wavelength Conversion Substance 40

The wavelength conversion substances 40 absorb at least a portion of theprimary light emitted from the light emitting element 10, and emitsecondary light having a different wavelength from that of the primarylight. This can produce a light emitting device 100 emitting white lightby mixing colors of the primary and secondary lights, for example. Forthe wavelength conversion substances 40, one of the specific exampleslisted below can be used singly, or two or more can be used incombination.

First Phosphor 41

The first phosphor 41 emits green to yellow light. The peak emissionwavelength of the first phosphor 41 is preferably in a range between 520nm and 560 nm from the perspective of emission efficiency and colormixing with the light emitted by another light source. Specific examplesof phosphors emitting green light include yttrium aluminum garnetphosphors (e.g., Y₃(Al, Ga)₅)₁₂:Ce), lutetium aluminum garnet phosphors(e.g., Lu₃(Al,Ga)₅)₁₂:Ce), terbium aluminum garnet phosphors (e.g.,Tb₃(Al,Ga)₅O₁₂:Ce), silicate phosphors (e.g., (Ba,Sr)₂SiO₄:Eu),chlorosiliate phosphors (e.g., (SiO₄)₄C₁₂:Eu), β-sialon phosphors (e.g.,Si_(6-z)Al_(z)O_(z)N_(8-z):Eu (0<z<4.2)), SGS phosphors (e.g.,SrGa₂S₄:Eu) and the like. Specific examples of phosphors emitting yellowlight include α-sialon phosphors (e.g., M_(z)(Si,Al)₁₂(O,N)16 (0<z≤2, Mis Li, Mg, Ca, Y, or a lanthanide element other than La and Ce, and thelike. Some of the phosphors emitting green light listed above can emityellow light. For example, the peak emission wavelengths of yttriumaluminum garnet phosphors can be shifted towards longer wavelengths bysubstituting a portion of Y with Gd, whereby yellow light can beemitted. Moreover, some of these can also emit orange light.

Second Phosphor 42

The second phosphor 42 emits red light. The peak emission wavelength ofthe second phosphor 42 is preferably in a range between 620 nm and 670nm from the perspective of emission efficiency and color mixing with thelight emitted by another light source. Specific examples of phosphorsemitting red light include nitrogen-containing calcium aluminosilicate(CASN or SCASN) phosphors (e.g., (Sr,Ca)AlSiN₃:Eu) and the like.Additional examples include manganese-activated fluoride phosphors,expressed by the general expression (I) A₂[M_(1-a)Mn_(a)F₆], wherein Ais at least one selected from the group consisting of K, Li, Na, Rb, Cs,and NH4, M is at least one element selected from the group consisting ofthe Group 4 elements and the Group 14 elements, and a satisfies 0<a<0.Representative examples of the manganese-activated fluoride phosphorsare magnesium-activated potassium fluorosilicate phosphors (e.g.,K₂SiF₆:Mn).

Cover Member 50

The cover member 50 has optical reflectance. The cover member 50preferably has optical reflectance of at least 70% at the peak emissionwavelength of the light emitting element 10 from the perspective offorward light extraction efficiency, more preferably at least 80%, evenmore preferably at least 90%. The cover member 50, moreover, ispreferably white. Accordingly, the cover member 50 is preferablyconstructed by containing a white pigment 55 in its base material 51.The cover member, furthermore, can contain similar fillers to thosedescribed above for the light transmissive member 20. The cover member50 is in liquid form before curing. The cover member 50 can be formed bytransfer molding, injection molding, compression molding, potting, orthe like.

Base Material 51 for the Cover Member 50

A resin can be used for the base material 51 for the cover member 50,and examples include silicone resins, epoxy resins, phenol resins,polycarbonate resins, acrylic resins, or their modified resins. Amongall, silicon and modified silicone resins are preferable because theyare highly heat and light resistant. Specific examples of siliconeresins include dimethyl silicone resin, phenyl-methyl silicone resin,and diphenyl silicone resin. The base material 51 can contain similarfillers to those listed above for the base material 30 for the lighttransmissive member 20.

White Pigment 55

For the white pigment 55, any one of titanium oxide, zinc oxide,magnesium oxide, magnesium carbonate, magnesium hydroxide, calciumcarbonate, calcium hydroxide, calcium silicate, magnesium silicate,barium titanate, barium sulfate, aluminum hydroxide, aluminum oxide, andzirconium oxide can be used singly, or two or more of these can be usedin combination. The particle shape of the white pigment 55 can besuitably selected, and can be crushed or irregular shape. From thefluidity perspective, a spherical shape is preferable. The particle sizeof the white pigment 55 can be, for example, in a range between about0.1 μm and about 0.5 The content of the white pigment 55 in the covermember 50 can be suitably selected. From the perspective of opticalreflectance and viscosity in the fluid state, the content is preferably,for example, in a range between 10 wt % and 70 wt %, more preferably ina range between 30 wt % and 60 wt %. “Wt %” refers to the ratio of thewhite pigment 55 weight to the total weight of the cover member 50.

Light Guide Member 60

The light guide member 60 bonds the light emitting element 10 and thelight transmissive member 20, and guide the light from the lightemitting element 10 to the light transmissive member 20. For the basematerial for the light guide member 60, at least one of silicone resins,epoxy resins, phenol resins, polycarbonate resins, acrylic resins, andtheir modified resins can be used. Among all silicone or modifiedsilicone resins are preferable because they are highly heat and lightresistant. Specific examples of silicone resins include dimethylsilicone resin, phenyl-methyl silicone resin, and diphenyl siliconeresin. The base material for the light guide member 60 can containsimilar fillers to those listed above for the base material 30 for thelight transmissive member 20. The light guide member 60 can be omitted.

Conductive Bonding Members 70

For the conductive bonding members 70, tin-bismuth, tin-copper,tin-silver, or gold-tin solder can be used. The conductive bondingmembers 70 may alternatively be sintered bodies of silver, gold, copper,platinum, aluminum, or palladium powder bound with a resin binder. Theseconductive bonding materials are in a paste form before heating, whichare melted by heating and subsequently solidified by cooling. Gold,silver, or copper bumps can alternatively be used as the conductivebonding members 70.

Wiring Substrate 80

The wiring substrate 80 includes at least a base 81 and wiring 85supported by the base 81. The wiring substrate 80 may suitably have anelectrically insulating protective film, such as solder resist,coverlay, or the like. For the wiring substrate 80, a rigid substrate ispreferable from the perspective of the rigidity of the light emittingdevice 100, but may alternatively be a flexible substrate.

Base 81

The base 81, in the case of a rigid substrate, can be constructed with aresin, fiber-reinforced resin, ceramic, glass, metal, paper, or thelike. Examples of resins or fiber-reinforced resins include epoxy, glassepoxy, bismaleimide triazine (BT), polyimide and the like. Examples ofceramics include aluminum oxide, aluminum nitride, zirconium oxide,zirconium nitride, titanium oxide, titanium nitride, or a mixture ofthese. Examples of metals include copper, iron, nickel, chromium,aluminum, silver, gold, titanium, or their alloys. The base 81, in thecase of a flexible substrate, can be constructed with polyimide,polyethyleneterephtharate, liquid crystal polymer, cycloolefin polymer,or the like.

Wiring 85

The wiring 85 is formed at least on the front face of the body, but canalso be formed on the interior and/or the lateral faces and/or the rearface of the base. The wiring 85 preferably includes element connectionterminal parts, i.e., lands, where the light emitting element 10 ismounted, external connection terminals to be connected to an externalcircuit, and lead wiring connecting these terminals. The wiring 85 canbe formed with copper, iron, nickel, tungsten, chromium, aluminum,silver gold, titanium, palladium, rhodium, or their alloys. These metalsand alloys can be formed as a single layer or multiple layers. Copper ora copper alloy is particularly preferable from the perspective of heatdissipation. The wiring 85 may have an outermost layer made of silver,platinum, aluminum, rhodium, gold or any of their alloys from theperspective of wettability of the conductive bonding members and/oroptical reflectance.

EXAMPLE

One example of the present disclosure will be explained in detail below.It goes without saying that the present disclosure is not limited to theexample discussed below.

Example 1

The light emitting device according to Example 1 is a side emission typeLED, 1.8 mm in length, 0.32 mm in width, and 0.70 mm in depth(thickness) having the construction of the light emitting device 100shown in FIGS. 1A and 1B.

The wiring substrate 80 is 1.8 mm in length, 0.32 mm in width, and 0.36in depth (thickness), and has a base 81 and a pair of wiring 85 formedon the base 81 which are arranged in the X direction. The base 81 is arectangular parallelepiped piece made of BT resin (e.g., HL832NSF typeLCA manufactured by Mitsubishi Gas Chemical Company). The pair of wiring85 is each composed of copper/nickel/gold layers from the base 81 side.The pair of wiring 85 each includes an element connection terminalformed on the front face near the middle in the x direction, and anexternal connection terminal on the rear face formed from the respectiveend portion of the front face of the base 81 in the X direction throughthe lateral face. The element connection terminals each has a projectionof the copper layer of 0.04 mm in depth (thickness).

A single light emitting element 10 is flip-chip mounted on the elementconnection terminals of the pair of wiring 85 via the conductive bondingmembers 70. The light emitting element 10 is a parallelepiped LED chip,1.1 mm in length, 0.2 mm in width and 0.12 mm in depth (thickness), andcapable of emitting blue light having a peak emission wavelength of 452nm. The light emitting element 10 is constructed by forming asemiconductor stack 13 stacking an n-type layer, an active layer, and ap-type layer of nitride semiconductor in that order on the rear face ofa sapphire substrate 15 (the third principal face 15 a). The conductivebonding members 70 are gold-tin solder (Au:Sn=79:21), which are each0.015 mm in depth (thickness).

On the front face of the light emitting element 10 (the fourth principalface 15 b of the substrate), the rear face of the light transmissivemember 20 (the first principal face 20 a) is bonded via a light guidemember 60. The light transmissive member 20 is a parallelepiped piece of1.21 mm in length, 0.24 mm in width, and 0.16 mm in depth (thickness)made of a base material 30, which contains as wavelength conversionsubstances an europium-activated β-sialon first phosphor 41 and amanganese-activated potassium fluorosilicate second phosphor 42. Morespecifically, the light transmissive member 20 is constructed bystacking three layers from the light emitting element 10 side: a layer201 composed of the base material 31 and the first phosphor 41, a layer202 composed of the base material 32 and the second phosphor 42, and alayer 203 made of the base material 33. The base material 30, i.e., thebase materials 31, 32, and 33 are all phenyl-methyl silicon resincontaining nano particles of silicon oxide as a filler. The light guidemember 60 is cured dimethyl silicone resin having 0.005 mm in depth(thickness). The light guide member 60 at least partially covers thelateral faces of the light emitting element 10.

On the front face of the substrate 80, the optically reflective covermember 50 is formed so as to surround of the lateral faces of the lightemitting element 10 and the light transmissive member 20 along theentire perimeter. The cover member 50, which is 1.35 mm in length and1.35 mm in width, is composed of a phenyl-methyl silicone resin basematerial 51 which contains 60 wt % of a white pigment 55 and titaniumoxide. The cover member 50 directly covers the lateral faces of thelight emitting element 10, the lateral faces 20 c of the lighttransmissive member 20, the lateral faces of the light guide member 60,and the lateral faces of the conductive bonding member 70. The frontface of the cover member 50 is substantially coplanar with the frontface of the light transmissive member 20. The two lateral faces of thecover member 50 perpendicular to the Y₊/Y⁻ direction are substantiallycoplanar with the two faces of the wiring substrate 80 perpendicular tothe Y₊/Y⁻ direction. The lateral faces of the cover member 50 and thewiring substrate 80 perpendicular to the Y⁻ direction is the mountingface of the light emitting device. Being defined by the cover member 50,the front face of the light transmissive member 20 (the second principalface 20 b) substantially forms the emission area of the light emittingdevice.

Such a light emitting device according to Example 1 was prepared asdescribed below. First, the wavelength conversion sheet 200 was preparedby successively adhering together the first sheet composed of the basematerial 31 and the first phosphor 41, the second sheet composed of thebase material 32 and the second phosphor 42, and the third sheetcomposed of the base material 33 in this order. Then, the wavelengthconversion sheet 200 was cut into small pieces each having the sizedescribed above using an ultrasonic cutter. Then, plural light emittingelements 10 arranged in the Y direction were flip-chip mounted on anaggregate substrate. Here, the aggregate substrate has a row of wiringsubstrates 80 arranged in the Y direction, and plural substrate areas inthe X direction partitioned by slits. The mounting of the light emittingelements 10 were performed by applying a paste form gold-tin solderserving as the conductive bonding members 70 to the element connectionterminals, mounting the light emitting elements 10 thereon, melting thegold-tin solder in a reflow furnace, and solidifying the solder. Next,the light guide member 60 in a liquid state was applied to the frontface of each light emitting element 10, the light transmissive member 20was placed thereon, and the light guide member 60 was cured by heatingin an oven. The processes described above formed plural light emittingstructures, each including a light emitting element 10, a light guidemember 60, and a light transmissive member 20 in this order, arranged inthe Y direction on the aggregate substrate. Next, by using a transfermolding die, a cover member 50 was formed on the aggregate substrate,embedding each of the light emitting structures with a parallelepipedcover member 50. Subsequently, the upper faces of the light transmissivemembers 20 were exposed by grinding the cover member 50 from the topusing a grinder. Lastly, by cutting the cover member 50 and theaggregate substrate between the light emitting structures along the Xdirection using a dicing machine, the light emitting devices 100 wereseparated into individual pieces.

Comparative Example 1

The light emitting device used as Comparative Example 1 was prepared ina similar manner to that for the light emitting device of Example 1except for using a light transmissive member, in which aneuropium-activated β-sialon phosphor and a manganese-activated potassiumfluorosilicate phosphor were uniformly dispersed in the entire basematerial, instead of the light transmissive member 20 used in Example 1.

Evaluation

By conducting aging tests on the light emitting devices of Example 1 andComparative Example 1 described above, and measuring the emissionchromaticity changes during reflow tests, where the devices were passedthrough a reflow oven, the reliability of each light emitting device wasevaluated. For the ageing tests the samples were aged for 500 hoursunder the conditions: forward current 20 mA, temperature 60° C., roomtemperature, and an ambient atmosphere. For the reflow tests, thesamples were passed through a reflow oven three times under theconditions: maximum temperature reached 260° C., retention time 10seconds, and an ambient atmosphere. FIGS. 4A and 4B show the results ofthese tests.

FIG. 3 is a graph showing the changes of the emission chromaticity ofthe light emitting devices of Example 1 and Comparative Example 1measured during the aging tests. FIGS. 4A and 4B are graphs showing thechanges of emission chromaticity x values and y values, respectively,measured during the reflow tests conducted on the light emitting devicesof Example 1 and Comparative Example 1. The chromaticity (value x andvalue y) and the chromaticity (value u′ and value v′) herein conform tothe (x, y) chromaticity diagram and the (u′, v′) chromaticity diagram,respectively, of the International Commission on Illumination (CIE). Itis understood from FIGS. 3, 4A, and 4B that the light emitting device ofExample 1 is highly reliable as shown by the small emission chromaticitychanges resulted as the tests progressed as compared to the lightemitting device of Comparative Example 1. It is inferred from this thatthe positioning of the wavelength conversion substances 40, particularlythe manganese-activated potassium fluorosilicate second phosphor 42, inthe light transmissive member 20 of Example 1 can moderate thedegradation thereof

The light emitting devices according to certain embodiments of thepresent disclosure can be utilized in backlights for liquid crystaldisplays, various types of lighting equipment, large displays, varioustypes of display devices for advertisements and destination signage,projectors, as well as image reading devices for digital video cameras,facsimiles, copiers, and scanners.

Although the present disclosure has been described with reference toseveral exemplary embodiments, it shall be understood that the wordsthat have been used are words of description and illustration, ratherthan words of limitation. Changes may be made within the purview of theappended claims, as presently stated and as amended, without departingfrom the scope and spirit of the disclosure in its aspects. Although thedisclosure has been described with reference to particular examples,means, and embodiments, the disclosure may be not intended to be limitedto the particulars disclosed; rather the disclosure extends to allfunctionally equivalent structures, methods, and uses such as are withinthe scope of the appended claims.

One or more examples or embodiments of the disclosure may be referred toherein, individually and/or collectively, by the term “disclosure”merely for convenience and without intending to voluntarily limit thescope of this application to any particular disclosure or inventiveconcept. Moreover, although specific examples and embodiments have beenillustrated and described herein, it should be appreciated that anysubsequent arrangement designed to achieve the same or similar purposemay be substituted for the specific examples or embodiments shown. Thisdisclosure may be intended to cover any and all subsequent adaptationsor variations of various examples and embodiments. Combinations of theabove examples and embodiments, and other examples and embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the description.

In addition, in the foregoing Detailed Description, various features maybe grouped together or described in a single embodiment for the purposeof streamlining the disclosure. This disclosure may be not to beinterpreted as reflecting an intention that the claimed embodimentsrequire more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive subject matter may bedirected to less than all of the features of any of the disclosedembodiments. Thus, the following claims are incorporated into theDetailed Description, with each claim standing on its own as definingseparately claimed subject matter.

The above disclosed subject matter shall be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments which fall within thetrue spirit and scope of the present disclosure. Thus, to the maximumextent allowed by law, the scope of the present disclosure may bedetermined by the broadest permissible interpretation of the followingclaims and their equivalents, and shall not be restricted or limited bythe foregoing detailed description.

What is claimed is:
 1. A method of producing a light emitting devicecomprising the steps of: preparing a wavelength conversion sheet bysuccessively adhering together a first sheet including a base materialand a first phosphor, a second sheet including a base material and asecond phosphor, and a third sheet including a base material in thisorder; preparing a light transmissive member by cutting the wavelengthconversion sheet into small pieces using an ultrasonic cutter; applyinga light guide member on a light emitting device and placing the lighttransmissive member on the light emitting device via the light guidemember; forming a cover member that embed the light emitting device, thelight guide member and the light transmissive member; and exposing anupper face of the light transmissive member by grinding the cover memberusing a grinder.
 2. The method of producing the light emitting deviceaccording to claim 1, further comprising the step of flip-chip mountingthe light emitting device on an aggregate substrate before the applyingand placing step.
 3. The method of producing the light emitting deviceaccording to claim 2, wherein the flip-chip mounting step includes thestep of applying a conductive bonding member on element connectionterminals of the aggregate substrate.
 4. The method of producing thelight emitting device according to claim 1, wherein the first phosphorincludes an europium-activated β-sialon phosphor.
 5. The method ofproducing the light emitting device according to claim 1, wherein thesecond phosphor includes an europium-activated β-sialon phosphor.